Previous work with cats has shown that functional changes in central auditory mechanisms follow sound-induced hearing loss or mechanical destruction of the cochleas. The functional changes include a virtual elimination of temporal summation for electrical stimulation of cochlear nucleus. A power function model, in contrast to an exponential model, has proved useful in describing temporal summation in normal-hearing and hearing-impaired human subjects. This same model can also characterize temporal summation data obtained with electrical stimulation of cochlear nucleus in normal-hearing cats and in cats with mechanically destroyed cochleas. The objective of the proposed work is to test the hypothesis that there are at least two sites of temporal summation--one of these sites being within the cochlea, while other summation processes reside in central auditory mechanisms. In normal-hearing cats, temporal summation will be measured using operant behavioral procedures both with acoustic stimuli and with electrical stimulation of auditory brainstem nuclei. In addition to type (auditory/electrical) and site (peripheral/central) of stimulation, sound-induced hearing-loss will be used to alter the operation of the summation processes. The use of a single species in the proposed work is important in order to probe the nature of the interrelated operations underlying auditory temporal summation. Knowledge concerning central involvement following hearing loss may prove to be of importance in the design of hearing aids, in the interpretation of evoked response data, and in the search for a useful cochlear prosthesis.